Downwasting of the debris-covered area of Lirung Glacier in Langtang Valley, Nepal Himalaya, from 1974 to 2010

Despite the widespread shrinkage of debris-covered Himalayan glaciers in recent decades, the underlying processes driving these changes are poorly understood. This study presents recent mass-balance data for the debris-covered area of Lirung Glacier in Langtang Valley, Nepal Himalaya, constrained using multi-temporal, remotely sensed digital elevation models calibrated to in situ GPS survey data, and surface-flow velocity data from phase-only correlation. The results indicate surface lowering of between −1.3 and −1.8 m a-1 during the study period (1974-2010), with accelerated glacier thinning after 2000. Similarly, we observed a decline in emergence velocity in the upper debris-covered area since 2000. We argue that this deceleration plays a key role in surface lowering, in contrast to declining surface mass balance, which we suggest is a residual effect of emergence velocity and surface lowering. Taken together, our findings indicate that the recent increase in surface lowering is attributable to the declining flux of ice from the upper to lower debris-covered areas of Lirung Glacier. Furthermore, this pattern suggests that downwasting of the upper debris-covered area will be augmented by a positive feedback between surface lowering and decelerated flow velocity.